The present invention relates to composite structures and, more particularly, to a composite member having increased resistance to delamination caused by interlaminar tensile stresses and to a method of making such a member.
Structures formed of composite materials are widely used in load-bearing structural applications. A composite structure is formed typically by laying a number of thin laminates or plies of fiber-matrix material atop one another to build a part of the desired shape and thickness, and then treating the part to cause the matrix material to bond the various plies together to form an integral structure. Each ply may have only unidirectional fibers, or may have multidirectional fibers. The fibers in different plies may have different orientations.
Composite structures have a number of advantages over non-composite structures. For instance, unlike isotropic materials such as metals, composites can be tailored to have different strength in different directions by varying the number of plies and the angular orientations of the different plies, so that the strength properties of the structure are matched to the load distribution which is expected to be experienced by the structure in use.
Composite materials, however, also have a disadvantage compared to isotropic materials, namely, that the matrix material which holds the fibers together is relatively weak in comparison to the fibers, and therefore composite materials tend to be less damage tolerant than isotropic materials such as metals. Once the matrix material is broken at a given location, such as by impact of the structure by an object, the fibers adjacent that location are no longer held in their desired orientations or xe2x80x9cfiber paths.xe2x80x9d Since the ability of the structure to withstand the expected loads depends on the strength of the fibers and on the fibers being oriented generally along the path of the loads, any deviation of the fibers from their desired orientations can result in substantial failure of the structure.
A particularly weak zone of composite laminates is the interface between two adjacent plies or laminates. There typically is a thin layer of matrix material separating the fibers in one laminate from the fibers in the adjacent laminate. Accordingly, this interface layer or xe2x80x9cboundary layerxe2x80x9d derives its strength solely from the strength of the matrix material, which, as previously noted, is relatively low compared to the strength of the fibers. For this reason, failure by xe2x80x9cdelamination,xe2x80x9d where adjacent laminates separate along their interface, is a particularly troublesome failure mode for composite laminate structures.
Techniques for strengthening composite laminates against delamination have been developed. One technique is to include xe2x80x9cthird axisxe2x80x9d fibers which extend between and through the laminates. For example, a method has been proposed wherein short fibers of carbon or wires of titanium are ultrasonically driven through one face of the laminate structure so that they extend in a thickness direction through the structure. Alternatively, stitching has been used as a means for holding the plies or laminates together and reducing the tendency toward delamination. However, these methods have the disadvantage that they tend to increase the stiffness of the structure, which is undesirable where natural resonant frequencies of the structure must be accurately ascertainable and controlled. Additionally, these methods are relatively cumbersome and expensive.
Moreover, it has been hypothesized, and verified by experimentation, that interlaminar tensile stresses between adjacent laminates are greatest at the free edges of a composite structure and rapidly decrease to near zero at the center. The known techniques for reinforcing composites, such as third axis fibers and stitching, have not taken into account this characteristic of the interlaminar tensile stress distribution.
Additionally, the third axis fiber technique and the stitching technique share the disadvantage that damage is done to the fibers of the composite structure when the third axis fibers or stitches are passed through the structure. Moreover, these reinforcing techniques cannot be used for reinforcing a composite structure formed from pre-cured laminates.
The present invention, in one embodiment, provides a composite member having a reinforcing wrap of fiber-matrix composite material which encircles the member or a portion thereof. The wrap is wrapped in a direction generally from one side edge surface of the member to the opposite side edge surface. The wrap at each side edge surface includes fibers which extend continuously from the upper face to the lower face. The composite laminates and wrap are bonded together by the matrix material to form an integral composite member having increased resistance to delamination by virtue of the composite wrap which provides additional strength in the third axis or Z-direction at the edge surfaces where interlaminar tensile stresses are greatest.
In a preferred embodiment, the composite wrap includes fibers that are oriented from about xe2x88x9260xc2x0 to about +60xc2x0 with respect to the wrap direction. The wrap preferably comprises an elongate tape of predetermined width, and is wrapped in multiple turns about the member. The multiple turns may be layered one atop another so as to form a multi-layered composite wrap encircling at least a portion of the member. Alternatively or additionally, the multiple turns may be staggered relative to one another along a length direction of the member so as to form a generally single-layered composite wrap encircling a lengthwise-extending portion of the member.
In another preferred embodiment of the invention, the composite member includes two portions that have a different number of layers of composite wrap for achieving different degrees of reinforcement of the two portions. For instance, where one portion of the member is determined to have relatively low interlaminar tensile stresses and another portion is determined to have relatively high interlaminar tensile stresses, the one portion may be wrapped with only a single layer of composite wrap, while the other portion may be wrapped with two or more layers of composite wrap.
In accordance with yet another embodiment of the invention, the member further includes fiber-matrix composite edge protectors which are applied to the edges of the member before the composite wrap is wrapped about the member. Each of the edge protectors covers one of the side edge surfaces of the member and extends partially along the upper and lower faces toward the opposite side edge surface of the member. The edge protectors preferably include fibers oriented from about xe2x88x9260xc2x0 to about +60xc2x0 relative to the wrap direction. The edge protectors may comprise a single ply of composite material or multiple plies.
The invention also provides methods for making a composite member. In accordance with one embodiment of the invention, a method includes a step of juxtaposing a plurality of fiber-matrix composite laminates each having opposite peripheral edges in facewise disposition with one another and with the peripheral edges in general alignment so as to form a composite member having upper and lower faces and a pair of opposite side edge surfaces which extend between the upper and lower faces. Next, a fiber-matrix composite reinforcing wrap is wrapped in one or more layers about at least a portion of the member such that the fiber-matrix composite reinforcing wrap is wrapped in a wrap direction which extends generally from one side edge surface to the other. The member is then treated to cause matrix bonding of the reinforcing wrap to the laminates so as to form an integral composite member. Where the laminates are not pre-cured, the treatment step also results in matrix bonding of the laminates to one another.
In accordance with a preferred embodiment, the method includes the further step of determining an interlaminar tensile stress distribution occurring at an interface between a selected pair of adjacent laminates in response to a defined load distribution applied to the member, and identifying regions of the member having relatively higher interlaminar tensile stresses. Then, the reinforcing wrap is encircled about those regions of relatively higher stresses. A relatively greater thickness (e.g., more layers) of reinforcing wrap may be applied to those regions having relatively higher stresses, and a relatively smaller thickness (e.g., fewer layers) of wrap may be applied to those regions having relatively lower stresses.
The invention thus provides a relatively simple and inexpensive method of making a composite member having increased resistance to delamination. The reinforcing wrap also improves the damage tolerance of the composite member, and protects the member from tool drops, scratches, and the like. A further advantage of the method and composite member of the invention is that the reinforcing wrap has relatively little impact on the bending and torsional stiffness of the member. In addition, the application of the reinforcing wrap of the present invention to a composite member can be tailored such that the thickness of the reinforcing wrap applied to various regions of the composite member varies in proportion to the level of stress to which the various regions will likely be subjected in use.